VARIATION

Living organisms differ from one another However there are some common traits

Variations are the phenotypic differences among the individuals of a species

Types of variations :1. continous variations2. discontinous variations

Continous variations A type of variations in which the differences in a character are not distinct but forms a continous range of intermediate phenotypes between two extreme

Examples :Weight, height Chest circumferences, body mass, hand span

The characteristics of a continous variations :1. no distinctive categories into which individuals can be placed 2. usually quantitative can be measured 3. controlled by large number of genes

4. significantly affected by a large number of genes 5. result of the combined effects of genes and the environment

Discontinous variations A type of variation in which the differences in a character can be divided into two or a few types of distinct phenotypes with no intermediate characteristics

Example :Four types of fingerprint patterns Blood group A, B, AB, OAbility to roll tongueThumb hyperextension

Characteristics of a discontinous variation:Distinctive categories in which individuals can be placedIndividual has the characteristics or does not have itQualitative cannot be measured

Characters are controlled by one gene or a small number of genesLargely unaffected by the environment

Discontinous variations are rare in humans but more common in plantsPlant flower, seed colour

Causes of variations Genetic factors Environmental factorsInteractions between genetic factors and environmental factors

Environmental variations cannot be inherited

Genetic variations Sources of variations :Sexual reproductionMutation

Variations geneticEnvironment Mutations Sexual reproductionChromosomalmutations Gene mutations Insertion deletionsubstitutionIndependent assortment Crossing overRandom fertilisation

Sexual ReproductionThree sources :1.crossing over2. independent assortment 3.random fertilisation

Crossing over - the exchange of genetic materials between the chromatids result in new, different genetic combinations of genes from the parent

Independent assortment during meiosis - during metaphase I homologous chromosomes arrange themselves randomly at equator

Independent assortment produces various genetic combinations in the gametes

Random fertilisation - each gamete (sperm or ovum) has different combination of genes - when the sperm and ovum fertilised it will produce a unique zygote

Genetic variations caused by mutationMutation a sudden permanent change in the nucleotide sequence of the DNA or in the amount of the DNA

Can involve a single nucleotide pair or a large region of a chromosomeRarely occurs Permanent and irreversible Occur in somatic cell or in gametes

Mutation can be cause by :Chemical or physical agent Mistakes that occur during DNA replicationUneven distribution of the chromosomes

Physical and chemical agents- X-ray, UV, gamma rays, nuclear radiation - penetrates the nucleus of the cell and damage the structure of the genes

- chemical mutagent :- benzene, formaldehyde, asbestos, carbon tetrachloride, mustard gas, tar, pesticides

Mistakes that occur during DNA replication- occurs when there is a change in the nucleotide sequence of the DNA molecule

Gene mutation occurs at a single locus on a chromosomes Involve :SubstitutionDeletionInsertion

G C A T C G A T T C GG C A T G A T T C GG C A T C C G A T T C GG C A T C G A T T A GDelete CInsert CRemove C replace with ABase deletionBase insertionBase substitution

Change in the sequence of bases usually result in a defective protein being synthesised or no protein is produced

Change in the DNA can lead to changes in cell function

Inherited disease caused by gene mutation :Sickle cell anaemia Albinism no enzyme responsible for the production of melanin (skin pigment )Polydactylism extra finger, or toe

Chromosomal mutation Caused by :1. increase or decrease in the number of chromosome2. change in the structure of chromosome

Increase or decrease in the number of chromosomes - homologous chromosomes or sister chromatids fail to separate during meiosis - result in the increase or decrease of chromosom number

Occurs in anaphase of meiosis I or II

The gametes produced would have 22 chromosomes or 24 chromosomes Down syndrome ( extra copy of chromosomes 21 )

23 24 47Zygote All cells in humans have 47 chromosomes

The probability of having a child with Down's syndrome increase with an increase in the age of the mother

The homologous chromosomes which fail to separate could be the sex chromosomes

Example : sperm 22 autosomes and no sex chromosomes This sperm is called O

Sperm can have two sex chromosomes

Normal gametes fuse with defective sperm or ovaResultant zygote : have a normal number of autosomes an abnormal number of sex chromosomes

Sex chromosomes of a defective sperm

O (none)

XX

YY

XYSex chromosomesof a normal ovum

X

X

X

X

Sex chromosomesof offspring

XO

XXX

XYY

XXYPhenotype

turner syndrome

trisomy X

XYY male

Klinefelter'ssyndrome

Sex chromosomes of a defective ovum

O (none)

O (none)

XX

XXSex chromosomesof a normal sperm

X

Y

X

Y

Sex chromosomesof offspring

XO

YO

XXX

XXYPhenotype

turner syndrome

Dies at embryo stage

trisomy X

Klinefelter'ssyndrome

Turner syndrome 2n-1 = 45 (44+XO)Hormone defeciencies prevent the XO females from menstruating No secondary sexual characteristics infertile

Short statureThick neckOnly have one X chromosomes - X-linked recessive disorder :Colour blindness

trisomy2n+1 = 47 (44+XXX) TallBelow normal level of intelligence Fertiles

Klinefelter 2n+1 = 47 (44 + XXY)Men that show mixed secondary sexual characteristics Partially breast developmentBroadening of the hipsSmall testes

Infertile low sperm count

XXY males Tall High testosterone Severe acneMore predisposed to criminal activities and behavioural problems

Another type of chromosomal mutation is the addition of one or more sets of chromosomes into an organisms polyploidy

Polyploid organisms:Triploids (3n) three sets of chromosomes of one type Tetraploid (4n) have four sets of chromosomes

Polyploidy is common in flowering plants Not occuring in animals Produce bigger fruit with more nutritional value

Change in the structure of the chromosomes Various radiation or organic chemicals can cause chromosomes to break

When chromosomes breaks the two broken end can reunite but do not rejoin in the same pattern as previously

Four types of chromosomal duplication DuplicationTranslocationDeletionInversion

Duplication

abc

defg

abc

dedefgOccurs when a broken segment of a chromosomes joins withthe other chromosomes of the homologous pair

abc

defg

abc

defg

abc

defg

abc

deqr

abc

defg

abc

defg

lmn

opqr

abc

defg

abc

defg

lmn

opfgTranslocation Occurs when a segment of a chromosome breaks off and joins a non homologous chromosomes

abc

defg

abc

dfgDeletion Occurs when one end of a chromosomeor an internal segment breaks off the two rejoin to form a chromosome defecient gene

abc

defg

abc

dfegBreak Inversion Occurs when a segment of a chromosome breaks loose, rotates 180 andrejoin in reverse direction- lead to altered gene activity

Environmental factors Environmental factors that cause variations :TemperatureLight intensityHumidityNutritionSoil fertility

Examples :1. flowering plant Hydrangea sp- acidic soil blue flower- alkaline soil pink flower

2. unbalanced diet - child may not reach to full height even he has gene for tallness balanced diet and carries weight :- achieve a physique of a bodybuilder

Identical twins :Same genotypeWill have different phenotype in different environment Well nourish environment and poorly nourish environment

Environmental factors cannot change the phenotype more than what has been determined by the genotype

The importance of variations in the survival of the species Provide necessary adaptations to the changing environment Enable organisms to survive more successully compared to other organisms

Produce diversity in organisms

Interactions between environmental factors and genetic factors Before industrialisation in England environment is unpolluted

Grey coloured Biston betularia(large number )Dark melanicBiston betularia(small number)Well camouflaged with the lichen covered tree trunksCamouflaged protectthem from betularia Not camoufaged by the lichen covered tree trunk Easily detected and eaten by predatorUnpolluted environment

During industrial revolution

Polluted environment Soot accumulated on the tree trunkLichen that live on the tree trunk died Grey coloured moths cannot camouflage themselves Their populationdecline Dark melanic populationincreased as they werecamouflaged by environment